Melt the butter in a deep, medium saucepan over medium-high heat until bubbling. Add the sugar and whisk until combined. Whisk in 1 1/3 cup of buttermilk, corn syrup, orange zest ( Tip: zest the orange over the pan to insure the oil gets into the caramel), and salt, stirring until dissolved. Bring to a boil. With tongs remove the orange zest. Cook, stirring occasionally, until the mixture is deep amber in color, about 10 minutes.
Remove from the heat, and pour in the remaining 2/3 cup buttermilk and vanilla. The caramel will bubble up from the addition of the buttermilk which works the same as adding baking soda. Whisk vigorously until the bubbling subsides and the mixture is dark and smooth.
Return the pan to medium-high heat and simmer for 3 to 5 minutes until the sauce thickens. Caramel sauce will thicken as it cools. Stopping when the sauce is thin will allow it to thicken to just the right consistency. Cool for about 5 minutes, serve warm.